Datasheet

 2014 Microchip Technology Inc. DS20005323A-page 7
HV9910C
3.0 APPLICATION INFORMATION
HV9910C is optimized to drive buck LED drivers using
open-loop, peak-current mode control. This method of
control enables fairly accurate LED current control
without the need for high side current sensing or the
design of any closed loop controllers. The IC uses very
few external components and enables both Linear and
PWM-dimming of the LED current.
A resistor connected to the RT pin programs the fre-
quency of operation (or the off-time). The oscillator pro-
duces pulses at regular intervals. These pulses set the
SR flip-flop in the HV9910C which causes the GATE
driver to turn on. The same pulses also start the blank-
ing timer, which inhibits the reset input of the SR flip flop
and prevents false turn-offs due to the turn-on spike.
When the FET turns on, the current through the induc-
tor starts ramping up. This current flows through the
external sense resistor, R
CS
, and produces a ramp volt-
age at the CS pin. The comparators are constantly
comparing the CS pin voltage to both the voltage at the
LD pin and the internal 250mV. Once the blanking timer
is complete, the output of these comparators is allowed
to reset the flip-flop. When the output of either one of
the two comparators goes high, the flip-flop is reset and
the GATE output goes low. The GATE goes low until
the SR flip-flop is set by the oscillator. Assuming a 30%
ripple in the inductor, the current sense resistor R
CS
can be set using:
Constant frequency peak current mode control has an
inherent disadvantage – at duty cycles greater than
0.5, the control scheme goes into subharmonic oscilla-
tions. To prevent this, an artificial slope is typically
added to the current sense waveform. This slope com-
pensation scheme will affect the accuracy of the LED
current in the present form. However, a constant off-
time peak current control scheme does not have this
problem and can easily operate at duty cycles greater
than 0.5. This control scheme also gives inherent input
voltage rejection, making the LED current almost
insensitive to input voltage variations. However, this
scheme leads to variable frequency operation and the
frequency range depends greatly on the input and out-
put voltage variation. Using HV9910C, it is easy to
switch between the two modes of operation by chang-
ing one connection (see Section 3.3 “Oscillator”).
3.1 Input Voltage Regulator
HV9910C can be powered directly from its V
IN
pin and
can work from 15 - 450VDC at its V
IN
pin. When a volt-
age is applied at the V
IN
pin, HV9910C maintains a
constant 7.5V at the V
DD
pin. This voltage is used to
power the IC and any external-resistor dividers needed
to control the IC. The V
DD
pin must be bypassed by a
low-ESR capacitor to provide a low impedance path for
the high frequency current of the output GATE driver.
HV9910C can also be operated by supplying a voltage
at the V
DD
pin greater than the internally regulated volt-
age. This will turn off the internal linear regulator of the
IC and the HV9910C will operate directly off the voltage
supplied at the V
DD
pin. This external voltage at the
V
DD
pin should not exceed 12V.
Although the V
IN
pin of the HV9910C is rated up to
450V, the actual maximum voltage that can be applied
is limited by the power dissipation in the IC. For exam-
ple, if an 8-lead SOIC HV9910C (junction to ambient
thermal resistance R
j-a
= 101°C/W) draws about I
IN
=
2.0mA from the V
IN
pin, and has a maximum allowable
temperature rise of the junction temperature limited to
T = 75°C, the maximum voltage at the V
IN
pin would
be:
In these cases, to operate HV9910C from higher input
voltages, a Zener diode can be added in series with the
V
IN
pin to divert some of the power loss from HV9910C
to the Zener diode. In the above example, using a 100V
Zener diode will allow the circuit to easily work up to
450V.
The input current drawn from the V
IN
pin is a sum of the
1.5mA (maximum) current drawn by the internal circuit
and the current drawn by the GATE driver. The GATE
driver depends on the switching frequency and the
GATE charge of the external FET.
In the above equation, f
s
is the switching frequency and
Q
g
is the GATE charge of the external FET, which can
be obtained from the data sheet of the FET.
3.2 Current Sense
The current sense input of HV9910C goes to the non-
inverting inputs of two comparators. The inverting ter-
minal of one comparator is tied to an internal 250mV
reference, whereas the inverting terminal of the other
comparator is connected to the LD pin. The outputs of
both these comparators are fed into an OR GATE and
R
CS
0.25V orV
LD

1.15 I
LED

------------------------------------=
Note: The Zener diode will increase the mini-
mum input voltage required to turn on the
HV9910C to 115V.
V
IN MAX
T
R
ja
-----------
1
I
IN
------

75C
101CW
-------------------------- -
1
2mA
------------ -

371V=
=
=
I
IN
1.5mA Q
g
f
s
+=